The present invention relates to the field of bypass turbojet engines and is aimed at attenuating the noise emitted by the engine, particularly the noise emitted by the fan.
A bypass turbojet engine for transport airplane propulsion comprises a fan, generally on the upstream side, delivering an annular airflow with a primary central annular part which supplies air to the engine that drives the fan and a secondary external annular part which is ejected to the atmosphere thereby providing a significant proportion of the thrust. The fan is contained inside a casing which delimits the secondary or bypass flow. A structural element of the engine known as the intermediate casing comprises a hub supporting the shafts that connect the various rotors, an outer annular casing and radial connecting arms connecting these casings together. The radial arms transmit some of the forces between the engine and its support. At the present time a ring of guide vanes is usually fitted, in the engines, between the fan rotor and the arms of the intermediate casing, in order to straighten the secondary flow along the axis of the engine. These vanes are generally known by their abbreviation OGV which stands for Outlet Guide Vanes.
Recently, engines have been developed in which the connecting arms act as flow straighteners straightening the flow downstream of the fan. This flow straightening function means that there needs to be a high number of arms and of OGVs, of the order of 30 to 50 of them in a bypass engine with 18 to 24 blades on the fan rotor, and therefore of a relatively small maximum thickness. In this example, the maximum thickness of the OGVs is 10 to 15 mm. In the case of certain arms it may exceed 100 mm. This arrangement, which also allows the flow straightener vanes to be kept away from the rotor blades thus improving fan efficiency by the order of 0.3 to 0.4 of a point in the case of engines with a high bypass ratio, has a favorable effect on the magnitude of the mechanisms of interaction between flow and cascade of guide vanes. It is at this cascade that noise is generated as a result of the impact of fluctuations in speed and in pressure on the vanes and which are caused by the unsteady flow. It should also be emphasized that the choice of the number of flow straightening vanes is already guided by acoustic criteria applied at the design stage with a view to reducing the noise at source. This criterion results in there being at least twice as many stator vanes as there are rotor blades in order to be able to cut out the acoustic modes generated by the interaction of the wake from the rotor on the stator at the rotor blade sweep frequency. This recommendation is applied fairly systematically to the design of any fan.